Nathan Schoppa - US grants

The overall goal of this proposal is to understand the functional mechanisms of inhibitory interneurons in the mammalian olfactory bulb. These interneurons, called granule cells and periglomerular (PG) cells, are often proposed to impact olfactory information processing in one of two ways, either by synchronizing the activity of the bulb's output neurons (mitral cells) or by mediating lateral inhibitory interactions between mitral cells. However, the functional mechanisms of the interneurons are not well-understood. Studies in the first two Specific Aims of this proposal use patch-clamp recordings from interneurons in olfactory bulb slices in order to evaluate two mechanistic aspects of interneurons that we predict are critical for function. These include the mechanisms of synaptic activation of interneurons by mitral cells (Aim 1) and mechanisms that couple the activity of different interneurons (Aim 2). In Specific Aim 3, recordings from pairs of mitral cells in slices will be made to test the functional role of interneurons in synchronizing action potential-firing in mitral cells. A key novel aspect of our studies is our electrical stimulation paradigm. While previous studies have examined interneuron mechanisms under static conditions, we examine their functional properties under dynamic, more physiological conditions produced by low frequency stimulation of afferent olfactory nerve (ON) fibers. The results of our studies will lead to a direct understanding of how synchronized activity is generated in the bulb, and also point toward possible mechanisms that could alter such activity following olfactory learning or during pathological conditions, such as Alzheimer's and Parkinson's disease. In addition, our mechanistic studies of synchronized activity could provide insight into mechanisms of epileptogenic dysfunction in other brain circuits.

? DESCRIPTION (provided by applicant) Glomeruli are the structures around which neurons are organized in the olfactory bulb. Each glomerulus carries information about a single odorant receptor in the nose, and has a specific population of glutamatergic mitral cells (MCs) and tufted cells (TCs) associated with it. The broad objective of this proposal is to build on provocative recent results from a variety of labs, including our own, pertaining to the cellular connectivity within a glomerulus. These studies suggest that most signaling between olfactory sensory neurons (OSNs) and MCs occurs through a multi-step path involving intermediary glutamatergic TCs (OSN-to-TC-to-MC), differing from the canonical model of direct OSN-to-MC excitation. In addition, glutamatergic signals from TCs are unusual, being mediated by long-range extrasynaptic transmission. These results are however controversial, as limited ultrastructural data suggest that OSNs are at least capable of making direct morphological contacts on MCs. In this application, we propose in Aim 1 to apply new approaches, including quantitative ultrastructural analyses, to establish whether the multi-step path is indeed the dominant signaling mechanism between OSNs and MCs. Aims 2 and 3 will then examine possible functional implications of a multi-step path for MCs using electrophysiological recordings in brain slices and awake behaving mice. The extrasynaptic nature of glutamate signaling from TCs, in particular, predicts that excitation of MCs should be highly non-linear, reflecting the accumulation of glutamate. For odor responses, this could, mean, for example, that MC responses to different odors are exceptionally sparse and display a steep dependence on odor concentration. A key tool in our functional studies will be connexin-36 knockout mice. Because OSN signaling onto MCs in these mice appears to be converted from multi-step to monosynaptic, they provide a potentially direct way to assess whether the multi-step activation path for MCs is causally related to specific functional properties. Taken together, our studies wil lead to important new information about the relationship between the structure and function of an olfactory circuit.